Dynamic Modeling and Analysis of 5‑PSS/UPU Parallel Mechanism with Elastically Active Branched Chains

To study the characteristics of the 5-prismatic–spherical–spherical(PSS)/universal–prismatic–universal(UPU)parallel mechanism with elastically active branched chains,the dynamics modeling and solutions of the parallel mechanism were investigated.First,the active branched chains and screw sliders were considered as spatial beam elements and plane beam element models,respectively,and the dynamic equations of each element model were derived using the Lagrange method.Second,the equations of the 5-PSS/UPU parallel mechanism were obtained according to the kinematic coupling relationship between the active branched chains and moving platform.Finally,based on the parallel mechanism dynamic equations,the natural frequency distribution of the 5-PSS/UPU parallel mechanism in the working space and elastic displacement of the moving platform were obtained.The results show that the natural frequency of the 5-PSS/UPU parallel mechanism under a given motion situation is greater than its operating frequency.The maximum position error is -0.096 mm in direction Y,and the maximum orientation error is -0.29°around the X-axis.The study provides important information for analyzing the dynamic performance,dynamic optimization design,and dynamic control of the 5-PSS/UPU parallel mechanism with elastically active branched chains.

Multi-objective Trajectory Planning Method based on the Improved Elitist Non-dominated Sorting Genetic Algorithm

Robot manipulators perform a point-point task under kinematic and dynamic constraints.Due to multi-degreeof-freedom coupling characteristics,it is difficult to find a better desired trajectory.In this paper,a multi-objective trajectory planning approach based on an improved elitist non-dominated sorting genetic algorithm(INSGA-II)is proposed.Trajectory function is planned with a new composite polynomial that by combining of quintic polynomials with cubic Bezier curves.Then,an INSGA-II,by introducing three genetic operators:ranking group selection(RGS),direction-based crossover(DBX)and adaptive precision-controllable mutation(APCM),is developed to optimize travelling time and torque fluctuation.Inverted generational distance,hypervolume and optimizer overhead are selected to evaluate the convergence,diversity and computational effort of algorithms.The optimal solution is determined via fuzzy comprehensive evaluation to obtain the optimal trajectory.Taking a serial-parallel hybrid manipulator as instance,the velocity and acceleration profiles obtained using this composite polynomial are compared with those obtained using a quintic B-spline method.The effectiveness and practicability of the proposed method are verified by simulation results.This research proposes a trajectory optimization method which can offer a better solution with efficiency and stability for a point-to-point task of robot manipulators.

Nanosecond laser conditioning of multilayer dielectric gratings for picosecond–petawatt laser systems

Multilayer dielectric gratings(MLDGs)are crucial for pulse compression in picosecond-petawatt laser systems.Bulged nodular defects,embedded in coating stacks during multilayer deposition,influence the lithographic process and performance of the final MLDG products.In this study,the integration of nanosecond laser conditioning(NLC)into different manufacturing stages of MLDGs was proposed for the first time on multilayer dielectric films(MLDFs)and final grating products to improve laser-induced damage performance.The results suggest that the remaining nodular ejection pits introduced by the two protocols exhibit a high nanosecond laser damage resistance,which remains stable when the irradiated laser fluence is more than twice the nanosecond-laser-induced damage threshold(nanosecond-LIDT)of the unconditioned MLDGs.Furthermore,the picosecond-LIDT of the nodular ej ection pit conditioned on the MLDFs was approximately 40%higher than that of the nodular defects,and the loss of the grating structure surrounding the nodular defects was avoided.Therefore,NLC is an effective strategy for improving the laser damage resistance of MLDGs.

Multilayer dielectric grating pillar-removal damage induced by a picosecond laser

Multilayer dielectric gratings typically remove multiple-grating pillars after picosecond laser irradiation;however,the dynamic formation process of the removal is still unclear.In this study,the damage morphologies of multilayer dielectric gratings induced by an 8.6-ps laser pulse were closely examined.The damage included the removal of a single grating pillar and consecutive adjacent grating pillars and did not involve the destruction of the internal high-reflection mirror structure.Comparative analysis of the two damage morphological characteristics indicated the removal of adjacent pillars was related to an impact process caused by the eruption of localized materials from the left-hand pillar,exerting impact pressure on its adjacent pillars and eventually resulting in multiple pillar removal.A finite-element strain model was used to calculate the stress distribution of the grating after impact.According to the electric field distribution,the eruptive pressure of the dielectric materials after ionization was also simulated.The results suggest that the eruptive pressure resulted in a stress concentration at the root of the adjacent pillar that was sufficient to cause damage,corresponding to the experimental removal of the adjacent pillar from the root.This study provides further understanding of the laser-induced damage behavior of grating pillars and some insights into reducing the undesirable damage process for practical applications.